Figure 2

Assessment of alveolar derecruitment by computed tomography (left panel) and pressure-volume curves (right panel). Image 1 shows a computed tomography (CT) section representative of the whole lung obtained at zero end-expiratory pressure (ZEEP). The dashed line separates poorly aerated and nonaerated lung areas (which appear in light gray and red, respectively, on image 2) from normally aerated lung areas (colored in dark gray on image 2 by a color-encoding system included in Lungview). Image 3 shows the same CT section obtained at a positive end-expiratory pressure (PEEP) of 15 cmH₂O. The delineation performed at ZEEP has been transposed on the new CT section in accordance with anatomical landmarks such as divisions of pulmonary vessels. Image 4 shows the same CT section with the color-encoding system, the overinflated lung areas appearing in white. Alveolar derecruitment was defined as the decrease in gas volume in poorly aerated and nonaerated lung regions after PEEP withdrawal. In the right panel, the pressure-volume (P–V) curves of the total respiratory system measured at ZEEP and a PEEP of 15 cmH₂O are represented. After determining the decrease in total gas volume resulting from PEEP withdrawal (ΔFRC), ΔFRC was added to each volume for constructing the P–V curve in PEEP conditions. The two curves were then placed on the same pressure and volume axis. Derecruitment volume was identified by a downward shift of the ZEEP P–V curve compared with the PEEP P–V curve and computed as the difference in lung volume between PEEP and ZEEP at an airway pressure of 15 cmH₂O.